On Ziv's rounding test

  • Authors:
  • Florent de Dinechin;Christoph Lauter;Jean-Michel Muller;Serge Torres

  • Affiliations:
  • École Normale Supérieure de Lyon, France;Université Pierre et Marie Curie Paris VI, France;CNRS, France;École Normale Supérieure de Lyon, France

  • Venue:
  • ACM Transactions on Mathematical Software (TOMS)
  • Year:
  • 2013

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Abstract

A very simple test, introduced by Ziv, allows one to determine if an approximation to the value f(x) of an elementary function at a given point x suffices to return the floating-point number nearest f(x). The same test may be used when implementing floating-point operations with input and output operands of different formats, using arithmetic operators tailored for manipulating operands of the same format. That test depends on a “magic constant” e. We show how to choose that constant e to make the test reliable and efficient. Various cases are considered, depending on the availability of an fma instruction, and on the range of f(x).